Functionally gradient composite article

ABSTRACT

A composite downhole article is disclosed. The article is selectively corrodible in a wellbore fluid. The article includes at least one corrodible core member comprising a first material that is corrodible in a wellbore fluid at a first corrosion rate. The article also includes at least one outer member disposed on the core member and comprising a second material that is corrodible in the wellbore fluid at a second corrosion rate, wherein the corrodible core member has a composition gradient or a density gradient, or a combination thereof, and wherein the first corrosion rate is substantially greater than the second corrosion rate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application contains subject matter related to the subject matterof co-pending patent application Attorney Docket Number WBI4-51865-USfiled on the same date as this application; which is assigned to thesame assignee as this application, Baker Hughes Incorporated of Houston,Tex., and incorporated herein by reference in its entirety.

BACKGROUND

Downhole drilling, completion and production operations often utilizewellbore components or tools that, due to their function, are onlyrequired to have limited service lives and must be removed from ordisposed of in the wellbore in order to recover the original size of thefluid pathway for use, including hydrocarbon production, CO₂sequestration, etc. Disposal of components or tools has conventionallybeen done by milling or drilling the component or tool out of thewellbore, which are generally time consuming and expensive operations.

The removal of components or tools by dissolution of degradablepolylactic polymers using various wellbore fluids has been proposed.However, these polymers generally do not have the mechanical strength,fracture toughness and other mechanical properties necessary to performthe functions of wellbore components or tools over the operatingtemperature range of the wellbore, therefore, their application has beenlimited.

Other degradable materials have been proposed including certaindegradable metal alloys formed from certain reactive metals in a majorportion, such as aluminum, together with other alloy constituents in aminor portion, such as gallium, indium, bismuth, tin and mixtures andcombinations thereof, and without excluding certain secondary alloyingelements, such as zinc, copper, silver, cadmium, lead, and mixtures andcombinations thereof. These materials may be formed by melting powdersof the constituents and then solidifying the melt to form the alloy, orusing powder metallurgy by pressing, compacting, sintering and the likea powder mixture of a reactive metal and other alloy constituent in theamounts mentioned. These materials include many combinations thatutilize heavy metals, such as lead, cadmium, and the like that may notbe suitable for release into the environment in conjunction with thedegradation of the material. Also, their formation may involve variousmelting phenomena that result in alloy structures that are dictated bythe phase equilibria and solidification characteristics of therespective alloy constituents, and that may not result in optimal ordesirable alloy microstructures, mechanical properties or dissolutioncharacteristics.

Therefore, the development of materials that can be used to formwellbore articles, such as components and tools, having the mechanicalproperties necessary to perform their intended function and then removedfrom the wellbore by controlled dissolution using wellbore fluids isvery desirable.

SUMMARY

In an exemplary embodiment, a composite downhole article is disclosed.The article is selectively corrodible in a wellbore fluid. The articleincludes at least one corrodible core member comprising a first materialthat is corrodible in a wellbore fluid at a first corrosion rate. Thearticle also includes at least one outer member disposed on the coremember and comprising a second material that is corrodible in thewellbore fluid at a second corrosion rate, wherein the corrodible coremember has a composition gradient or a density gradient, or acombination thereof, and wherein the first corrosion rate issubstantially greater than the second corrosion rate.

In another exemplary embodiment, another composite downhole article isdisclosed. The article is also selectively corrodible in a wellborefluid. The article includes at least one core member comprising a firstmaterial that is corrodible in a wellbore fluid at a first corrosionrate. The article also includes at least one corrodible outer memberdisposed on the core member and comprising a second material that iscorrodible in the wellbore fluid at a second corrosion rate, wherein thecorrodible outer member has a composition gradient or a densitygradient, or a combination thereof, and wherein the second corrosionrate is substantially greater than the first corrosion rate.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings wherein like elements are numbered alikein the several Figures:

FIG. 1 is a cross-sectional view of an exemplary embodiment of adownhole article as disclosed herein;

FIG. 2 is a cross-sectional view of section 2 of FIG. 1 illustrating anembodiment of a gradient portion as disclosed herein;

FIG. 3 is a cross-sectional view of another embodiment of a gradientportion as disclosed herein;

FIG. 4 is a cross-sectional view of a second exemplary embodiment of adownhole article as disclosed herein;

FIG. 5 is a cross-sectional view of a third exemplary embodiment of adownhole article as disclosed herein;

FIG. 6 is a cross-sectional view of a fourth exemplary embodiment of adownhole article as disclosed herein;

FIG. 7 is a cross-sectional view of a fifth exemplary embodiment of adownhole article as disclosed herein;

FIG. 8 is a cross-sectional view of a sixth exemplary embodiment of adownhole article as disclosed herein;

FIG. 9 is a cross-sectional view of a seventh exemplary embodiment of adownhole article as disclosed herein;

FIG. 10 is a cross-sectional view of an eighth exemplary embodiment of adownhole article as disclosed herein;

FIG. 11 is a cross-sectional view of a ninth exemplary embodiment of adownhole article as disclosed herein;

FIG. 12 is a cross-sectional view of a tenth exemplary embodiment of adownhole article as disclosed herein;

FIG. 13 is a flow diagram of a method of making a downhole article asdisclosed herein;

FIG. 14 is a flow diagram of a second method of making a downholearticle as disclosed herein;

FIG. 15 is a flow diagram of a method of using a downhole article asdisclosed herein;

FIG. 16 is a cross-sectional view of a coated metallic powder used tomake a nanomatrix composite powder compact as disclosed herein;

FIG. 17 is a cross-sectional view of a nanomatrix composite powdercompact as disclosed herein; and

FIG. 18 is a cross-sectional view of a precursor nanomatrix compositepowder compact as disclosed herein.

DETAILED DESCRIPTION

Referring to the FIGS. 1-12, a composite downhole article 10 isdisclosed. The composite downhole article 10 may include any one ofvarious downhole tools and components. These downhole tools andcomponents may include various diverter balls 12, ball seats 14, plugs16, plug seats 18, disks 20, darts 21, sleeves 22, tubular sections 23and the like. The composite downhole article 10 is selectivelycorrodible in a predetermined wellbore fluid 24. The composite downholearticles 10 may be selectively removed using the predetermined wellborefluid 24. Alternately, they may be reconfigured from one shape toanother shape or one size to another size using the predeterminedwellbore fluid 24 to selectively corrode a portion of the article 10.Combinations of these features are also possible, such as, for example,reconfiguration of the article from one shape to another shape or onesize to another size followed by removing the article 10 from thewellbore using the predetermined wellbore fluid 24, or a combination ofpredetermined wellbore fluids 24. The composite downhole articles 10described herein include a functionally gradient composite material thatincludes a rapidly corrodible metallic portion 26 and a more corrosionresistant portion 28 that is corrodible at a much slower rate in thepredetermined wellbore fluid 24. In certain embodiments, the compositedownhole article 10 may include a tough, selectively and rapidlycorrodible metallic core member 30 or substrate comprising a firstmaterial 32 that comprises the corrodible metallic portion 26 and thatis protected by hard and corrosion resistant outer member 40 comprisinga second material 42 that comprises the more corrosion resistant portion28. In other embodiments, the arrangement may be reversed and thecomposite downhole article 10 may include a tough, selectively andrapidly corrodible metallic outer member 50 or substrate comprising thefirst material 32 that comprises the corrodible metallic portion 26 thatencompasses a hard and corrosion resistant core member 60 comprising asecond material 42 that comprises the more corrosion resistant portion28. The corrodible metallic portion 26 may include a functionallygradient portion 70 that includes a functionally gradient material 70disposed between the first material 32 of corrodible metallic portion 26and the second material 42 of more corrosion resistant portion 28. Sucha structure enables the tool to resist corrosion during use of thearticle, such as tool operation, while also allowing rapidreconfiguration or removal when the core material is exposed to thepredetermined wellbore fluid. The gradient portion 70 may be used, forexample, to provide a microstructural transition between the firstmaterial 32 and the second material 42, since these materials may havesubstantially different metallurgical and mechanical properties. Thecorrodible metallic portion 26 may be formed from a nanomatrix compositematerial as disclosed herein. The relatively more corrosion resistantportion 28 may be formed from any suitable material that is morecorrosion resistant than the corrodible metallic portion 26, preferablysubstantially more corrosion resistant, and more particularly mayinclude materials that exhibit high hardness and wear resistance, forexample.

Referring to FIG. 1, in an exemplary embodiment, the composite downholearticle includes, at least one corrodible core member 30 comprising afirst material 32 that is corrodible in a wellbore fluid at a firstcorrosion rate. The composite downhole article 10 also includes at leastone outer member 40 disposed on the core member 30 and comprising asecond material 42 that is corrodible in the wellbore fluid at a secondcorrosion rate, wherein the corrodible core member 30 has a gradientportion 70 that includes a composition gradient or a density gradient,or a combination thereof, and wherein the first corrosion rate issubstantially greater than the second corrosion rate.

The outer member 40 may have any suitable form or thickness. In oneembodiment, the outer member 40 comprises a layer that is disposed onthe core member 30 by direct deposition of the second material 42 on anouter portion or surface 36 of the gradient portion 70 of the coremember 30 or alternately, on an outer portion or surface of a separatelyformed gradient portion 70 that is disposed on the core member 30.Various deposition methods may be employed, such as plating, sputteringand other thin film deposition techniques, cladding, compacting apowder, thermal spraying, or laser fusion of a powder as describedherein. The outer member 40 may also be formed as a separate member andattached to the outer portion 36 of the core member 30 by any suitableattachment method including those described herein. For example, theouter member 40 may be formed as a powder compact including as ananomatrix powder compact as described herein and then attached to theouter portion of the core member 30 by a suitable attachment method.Suitable attachment methods include isostatic pressing, diffusionbonding, thermal molding, welding, brazing, adhesives and the like. Theouter member 40 may also be formed in one or more portions or sectionswhich are attached to one another so as to encompass the core member 30,either with or without direct attachment to the core member 30. In anexemplary embodiment, outer member 40 may be formed as two thinhemispherical halves that may be placed around a substantially sphericalcore member 30 such that the hemispherical halves 33 press against thecore member 40 followed by, for example, joining the hemispheres by ajoint, such as a weld joint 35, around their adjoining peripheries so asto encompass the core member 30. The outer member 40 may have anysuitable thickness necessary to perform the wellbore operation oroperations of the article 10 with which it is associated. In anexemplary embodiment, the outer member 40 includes a relatively thinlayer disposed on the core member 30, and more particularly may have athickness of up to about 10 mm, and more particularly about 1 mm toabout 5 mm, and even more particularly about 0.1 mm to about 2 mm. Theouter member may also comprise a deposited thin film, and may have athickness that is 500 microns or less, and more particularly 100 micronsor less, and even more particularly 10 microns or less.

In certain embodiments, the core member 30 may be completely orpartially encompassed by the outer member 40, such as examples where theouter member 40 comprises an outer layer that completely or partiallyencompasses the core member 30. In other embodiments, the outer member40 may only be applied to a portion or portions of the core member 30,such as those which are exposed to the wellbore fluid 24. In oneembodiment, the article 10 comprises a substantially spherical diverterball 12 as illustrated in FIG. 1. The corrodible core member 30 issubstantially spherical and the outer member 40 is a substantiallyspherical layer disposed on the core member as illustrated in FIG. 1with the gradient portion 70 disposed between them. In anotherembodiment, the article 10 comprises a cylindrical plug 16 asillustrated in FIG. 4. The corrodible core member 30 is substantiallycylindrical and the outer member 40 comprises an encompassing layerdisposed on the core member 30. In yet another embodiment, the article10 comprises a hollow cylindrical sleeve 22 as illustrated in FIG. 5.The core member 30 comprises a hollow cylinder disposed about alongitudinal axis and the outer member 40 comprises a layer disposed onthe core member 30 and gradient portion 70. The 22 sleeve may alsocomprise a seat on one or both ends, such as a tapered ball seat 14, ora plug seat 18 as illustrated in FIG. 7. In still another embodiment,the article 10 may include a cylindrical disk 20 as illustrated in FIG.6. The core member 30 comprises a cylindrical disk and the outer member40 comprises a layer disposed on the core member 30 and gradient portion70. In another embodiment, the article 10 may include a dart 21 thatinclude a cylindrical disk portion 27 and a frustoconical tail portion29 as illustrated in FIG. 8. The frustoconical tail portion 29 maycomprise a plurality of tapered fins 31 that are radially spaced aboutthe longitudinal axis 33. The core member 30 comprises a cylindricaldisk and the outer member 40 comprises a layer disposed on the coremember 30 and gradient portion 70. In still another embodiment, thearticle 10 may include a cylindrical tubular section 23, such as may beused to form a section of a wellbore casing as illustrated in FIG. 9.One of ordinary skill will recognize that downhole tools or componentscomprising the article 10 shapes described above may be use in variousdrilling, completion and production operations, and these forms may alsoinclude various features 25 incorporated therein, such as various holes,slots, shoulders, grooves, ribs and the like as illustrated in FIG. 9 inconjunction with a tubular section 23. These shape forms may also benested within one another, such that a plurality of spherical balls orcylinders or sleeves as described above may be nested within one anotherand have progressively larger or smaller sizes. Articles 10 as disclosedherein having different shape forms mentioned herein may also be nestedwithin one another, such as a smaller ball 12 nested within a largerball 12 as illustrated in FIG. 11, and a smaller ball 12 nested within alarger plug 16 as illustrated in FIG. 12, or vice versa.

The corrodible core member 30 comprises a selectively corrodible firstmaterial 32. The first material 32 may include a metallic material thatmay be selectively and rapidly corroded by the predetermined wellborefluid. More particularly, the selectively corrodible metallic materialmay include various metallic nanomatrix composite materials as describedin commonly owned, co-pending U.S. patent application Ser. No.12/633,682 filed on Dec. 8, 2009 and Ser. No. 12/913,310 filed on Oct.27, 2010, which are incorporated herein by reference in their entirety.Referring to FIG. 16, the nanomatrix composites are compacts may beformed from a metallic powder 110 that includes a plurality of metallic,coated powder particles 112. Powder particles 112 may be formed toprovide a powder 110, including free-flowing powder, that may be pouredor otherwise disposed in all manner of forms or molds (not shown) havingall manner of shapes and sizes and that may be used to fashion precursorpowder compacts 100 (FIG. 19) and powder compacts 200 (FIG. 18), asdescribed herein, that may be used as, or for use in manufacturing,various articles of manufacture, including various wellbore tools andcomponents.

Each of the metallic, coated powder particles 112 of powder 10 includesa particle core 114 and a metallic coating layer 116 disposed on theparticle core 114. The particle core 114 includes a core material 118.The core material 118 may include any suitable material for forming theparticle core 114 that provides powder particle 112 that can be sinteredto form a lightweight, high-strength powder compact 200 havingselectable and controllable dissolution characteristics. In oneembodiment, suitable core materials include electrochemically activemetals having a standard oxidation potential greater than or equal tothat of Zn, and in another embodiment include Mg, Al, Mn, Fe or Zn, oralloys of these metals, or a combination thereof. Core material 118 mayalso include other metals that are less electrochemically active than Znor non-metallic materials, or a combination thereof. Suitablenon-metallic materials include ceramics, composites, glasses or carbon,or a combination thereof. Core material 118 may be selected to provide ahigh dissolution rate in a predetermined wellbore fluid, but may also beselected to provide a relatively low dissolution rate, including zerodissolution, where dissolution of the nanomatrix material causes theparticle core 114 to be rapidly undermined and liberated from theparticle compact at the interface with the wellbore fluid, such that theeffective rate of dissolution of particle compacts made using particlecores 114 of these core materials 118 is high, even though core material118 itself may have a low dissolution rate, including core materials 120that may be substantially insoluble in the wellbore fluid.

Each of the metallic, coated powder particles 112 of powder 110 alsoincludes a metallic coating layer 116 that is disposed on particle core114. Metallic coating layer 116 includes a metallic coating material120. Metallic coating material 120 gives the powder particles 112 andpowder 110 its metallic nature. Metallic coating layer 116 is ananoscale coating layer. In an exemplary embodiment, metallic coatinglayer 116 may have a thickness of about 25 nm to about 2500 nm. Thethickness of metallic coating layer 116 may vary over the surface ofparticle core 114, but will preferably have a substantially uniformthickness over the surface of particle core 114. Metallic coating layer116 may include a single layer or a plurality of layers as a multilayercoating structure. Metallic coating material 120 may include anysuitable metallic coating material 120 that provides a sinterable outersurface 121 that is configured to be sintered to an adjacent powderparticle 112 that also has a metallic coating layer 116 and sinterableouter surface 121. In an exemplary embodiment of a powder 110, particlecore 114 includes Mg, Al, Mn, Fe or Zn, or alloys thereof, or acombination thereof, as core material 118, and more particularly mayinclude pure Mg and Mg alloys, and metallic coating layer 116 includesAl, Zn, Mn, Mg, Mo, W, Cu, Fe, Si, Ca, Co, Ta, Re, or Ni, or alloysthereof, or an oxide, nitride or a carbide thereof, or a combination ofany of the aforementioned materials as coating material 120. Powder 110may also include an additional or second powder 30 interspersed in theplurality of powder particles 112, as illustrated in FIG. 16. In anexemplary embodiment, the second powder 130 includes a plurality ofsecond powder particles 132. These second powder particles 132 may beselected to change a physical, chemical, mechanical or other property ofa powder particle compact 200 formed from powder 110 and second powder130, or a combination of such properties. In an exemplary embodiment,the property change may include a gradient in composition or density, ora combination thereof, in gradient portion 70. Second powder particles132 may be uncoated or coated with a metallic coating layer 136. Whencoated, including single layer or multilayer coatings, the coating layer136 of second powder particles 132 may comprise the same coatingmaterial 140 as coating material 120 of powder particles 112, or thecoating material 140 may be different. The second powder particles 132(uncoated) or particle cores 134 may include any suitable material toprovide the desired benefit, including many metals. In an exemplaryembodiment, when coated powder particles 112 comprising Mg, Al, Mn, Feor Zn, or alloys thereof, or a combination thereof are employed,suitable second powder particles 32 may include Ni, W, Cu, Co or Fe, oralloys thereof, or a combination thereof, for example.

As used herein, the use of the term substantially-continuous cellularnanomatrix 216 does not connote the major constituent of the powdercompact, but rather refers to the minority constituent or constituents,whether by weight or by volume. This is distinguished from most matrixcomposite materials where the matrix comprises the majority constituentby weight or volume. The use of the term substantially-continuous,cellular nanomatrix is intended to describe the extensive, regular,continuous and interconnected nature of the distribution of nanomatrixmaterial 220 within powder compact 200. As used herein,“substantially-continuous” describes the extension of the nanomatrixmaterial throughout powder compact 200 such that it extends between andenvelopes substantially all of the dispersed particles 214.Substantially-continuous is used to indicate that complete continuityand regular order of the nanomatrix around each dispersed particle 214is not required. For example, defects in the coating layer 116 overparticle core 114 on some powder particles 112 may cause bridging of theparticle cores 114 during sintering of the powder compact 200, therebycausing localized discontinuities to result within the cellularnanomatrix 216, even though in the other portions of the powder compactthe nanomatrix is substantially continuous and exhibits the structuredescribed herein. As used herein, “cellular” is used to indicate thatthe nanomatrix defines a network of generally repeating, interconnected,compartments or cells of nanomatrix material 220 that encompass and alsointerconnect the dispersed particles 214. As used herein, “nanomatrix”is used to describe the size or scale of the matrix, particularly thethickness of the matrix between adjacent dispersed particles 214. Themetallic coating layers that are sintered together to form thenanomatrix are themselves nanoscale thickness coating layers. Since thenanomatrix at most locations, other than the intersection of more thantwo dispersed particles 214, generally comprises the interdiffusion andbonding of two coating layers 116 from adjacent powder particles 112having nanoscale thicknesses, the matrix formed also has a nanoscalethickness (e.g., approximately two times the coating layer thickness asdescribed herein) and is thus described as a nanomatrix. Further, theuse of the term dispersed particles 214 does not connote the minorconstituent of powder compact 200, but rather refers to the majorityconstituent or constituents, whether by weight or by volume. The use ofthe term dispersed particle is intended to convey the discontinuous anddiscrete distribution of particle core material 218 within powdercompact 200.

The equiaxed morphology of the dispersed particles 214 and cellularnetwork 216 of particle layers results from sintering and deformation ofthe powder particles 112 as they are compacted and interdiffuse anddeform to fill the interparticle spaces 115. The sintering temperaturesand pressures may be selected to ensure that the density of powdercompact 200 achieves substantially full theoretical density. Referringto FIG. 17, sintered powder compact 200 may comprise a sinteredprecursor powder compact 100 that includes a plurality of deformed,mechanically bonded powder particles as described herein. Precursorpowder compact 100 may be formed by compaction of powder 110 to thepoint that powder particles 112 are pressed into one another, therebydeforming them and forming interparticle mechanical or other bondsassociated with this deformation sufficient to cause the deformed powderparticles 112 to adhere to one another and form a green-state powdercompact having a green density that may be varied and is less than thetheoretical density of a fully-dense compact of powder 110, due in partto interparticle spaces 115. Compaction may be performed, for example,by isostatically pressing powder 110 at room temperature to provide thedeformation and interparticle bonding of powder particles 112 necessaryto form precursor powder compact 100.

Sintered and dynamically forged powder compacts 200 that includedispersed particles 214 comprising Mg and nanomatrix 216 comprisingvarious nanomatrix materials as described herein have demonstrated anexcellent mechanical strength and low density. Dynamic forging as usedherein means dynamic application of a load at temperature and for a timesufficient to promote sintering of the metallic coating layers 16 ofadjacent powder particles 12, and may preferably include application ofa dynamic forging load at a predetermined loading rate for a time and ata temperature sufficient to form a sintered and fully-dense powdercompact 200. In an exemplary embodiment where particle cores 14 includedMg and metallic coating layer 16 included various single and multilayercoating layers as described herein, such as various single andmultilayer coatings comprising Al, the dynamic forging was performed bysintering at a temperature, T_(S), of about 450° C. to about 470° C. forup to about 1 hour without the application of a forging pressure,followed by dynamic forging by application of isostatic pressures atramp rates between about 0.5 to about 2 ksi/second to a maximumpressure, P_(S), of about 30 ksi to about 60 ksi, which resulted inforging cycles of 15 seconds to about 120 seconds.

Powder compacts 200 that include dispersed particles 214 comprising Mgand nanomatrix 216 comprising various nanomatrix materials 220 describedherein have demonstrated room temperature compressive strengths of atleast about 37 ksi, and have further demonstrated room temperaturecompressive strengths in excess of about 50 ksi. Powder compacts 200 ofthe types disclosed herein are able to achieve an actual density that issubstantially equal to the predetermined theoretical density of acompact material based on the composition of powder 110, includingrelative amounts of constituents of particle cores 114 and metalliccoating layer 116, and are also described herein as being fully-densepowder compacts. Powder compacts 200 comprising dispersed particles thatinclude Mg and nanomatrix 216 that includes various nanomatrix materialsas described herein have demonstrated actual densities of about 1.738g/cm³ to about 2.50 g/cm³, which are substantially equal to thepredetermined theoretical densities, differing by at most 4% from thepredetermined theoretical densities. Powder compacts 200 comprisingdispersed particles 214 that include Mg and cellular nanomatrix 216 thatincludes various nanomatrix materials as described herein demonstratecorrosion rates in 15% HCl that range from about 4750 mg/cm²/hr to about7432 mg/cm²/hr. This range of response provides, for example the abilityto remove a 3 inch diameter ball formed from this material from awellbore by altering the wellbore fluid in less than one hour.

The outer member 40 is disposed on the core member 30 and includes asecond material 42 that is corrodible in the predetermined wellborefluid 24 at a second corrosion rate. The first corrosion rate of thefirst material 32 is substantially greater than the second corrosionrate of the second material 42 as described herein. The second material42 of the outer member 40 may be any suitable material, includingvarious metals, polymers or ceramics, or composites thereof, or othercombinations thereof. Suitable metals include alloys typically employedin tubular products used in a wellbore environment, including variousgrades of steel, particularly various grades of stainless steel. Othersuitable alloys include Fe-base, Ni-base and Co-base alloys andsuperalloys. Suitable polymers may include any polymer that provides lowpermeability to the predetermined wellbore fluid 24 for a timesufficient to function as second material 42 as described herein.Suitable polymers include various fluoropolymers and polyurethanes.Suitable ceramics may include metal carbides, oxides or nitrides, orcombinations thereof, including tungsten carbide, silicon carbide, boroncarbide, alumina, zirconia, chromium oxide, silicon nitride or titaniumnitride.

In one embodiment, the difference in the corrosion rates of the firstmaterial 32 and the second material 42 allows the selectively corrodibledownhole article 10 to be utilized for its intended purpose, such as aspecific wellbore operation, in the presence of the predeterminedwellbore fluid 24 and provides an operating lifetime or critical servicetime in the predetermined wellbore fluid 24 that is sufficient toperform the wellbore operation. In another exemplary embodiment, thedifference in corrosion rates allows the downhole article 10 to beutilized for its intended purpose, such as a specific wellboreoperation, without being exposed to the predetermined wellbore fluid 24,and once the wellbore operation is completed the predetermined wellborefluid may be introduced to selectively corrode the article 10. Examplesof the rapid corrosion rates of the first material 32 are providedabove. The second corrosion rate of the second material 42 in thewellbore fluid may be any suitable rate that is lower than the firstcorrosion rate, more particularly it may be lower by about one to aboutten orders of magnitude, and more particularly by about one to aboutthree orders of magnitude. This may include corrosion rates of about0.001 mg/cm²/hr to about 1.0 mg/cm²/hr.

As illustrated in the Figures, and more particularly in FIGS. 2 and 3,the corrodible core member 32 has a gradient portion 70 that has acomposition gradient or a density gradient, or a combination thereof. Inone embodiment, the gradient portion 70 includes a composition gradientor density gradient, or combination thereof, that includes one or moretransition layers disposed between the corrodible core member and theouter member. This layer or layers may be used for any suitable purpose,including, for example, to provide a transition between the firstmaterial and second material where these materials have differentmaterial properties, such as different crystal structures, coefficientsof thermal expansion and the like, in order to reduce the stresses atthe interface between them and promote the adherence of the outer member40 to the core member 30. This layer or layers may also be used tocontrol the density of the article 10 by providing a region in which thefirst material 32 of the core member may be adjusted by the addition ofa controlled amount of another material, such as an amount of the secondmaterial 42, in order to provide the article 10 with a predetermineddensity. This layer or layers may also be used to control the corrosionrate of the article 10 by providing a layer or layers that have adifferent corrosion rate than that of the first material 32 of the coremember 30 or the second material 42 of the outer member 40, such as acorrosion rate that lies between the corrosion rates of the firstmaterial 32 and the second material 42. While this gradient portion 70is described above as a composition gradient or density gradient, orcombination thereof, of the core member 30 it will be understood thatthe gradient portion 70 may also be associated with the outer member 40,and further, may be considered to be a separate gradient member 72disposed between the outer member 40 and the core member 30. While thecomposition gradient or density gradient, or combination thereof, maycomprise a layer or plurality of layers disposed uniformly about thecore member 30, it may also be disposed non-uniformly as a portion 70 orregion between the core member 30 and the outer member 40, and may beused, for example, to provide a varying weight distribution within thearticle 10, including various non-axisymmetric weight distributions. Assuch, the composition gradient or density gradient, or combinationthereof, may be used, for example, to orient or preposition the articleas it descends in the wellbore through a wellbore fluid by non-uniformlyweighting a specific portion of the article 10.

Gradient portion 70 and the associated composition gradient or densitygradient, or a combination thereof, may be established in any suitablemanner. In an exemplary embodiment a composition gradient may beestablished by disposing a layer that includes a powder compact of apowder mixture of the first material 32 and another material, such asthe second material 42, between the core member and the outer member.Even where the core member 30 and the gradient portion 70 or layer thatincludes the composition gradient are compacted to full theoreticaldensity, such an arrangement provides a composition gradient and adensity gradient so long as the first material and the other materialare different materials having different densities. For example, if thecore member 30 is formed by compacting a powder comprising magnesiumalloy particle cores having aluminum or aluminum alloy nano layers toform a nanomatrix composite comprising dispersed magnesium particles inan aluminum or aluminum alloy nanomatrix as described herein, acomposition gradient may be formed in gradient portion 70 by compactinga mixture of the same aluminum coated magnesium powder particles used toform the core member 30 with nanoparticles or microparticles of anothermetal or metal alloy, such as particles of the second material 42. Whilea composition gradient may be formed by using the second material 42 ofthe outer member 40, a density gradient may also be formed using anyother material, including second material 42 that has a densitydifferent from the first material 32. The other material used to formthe composition gradient may be any suitable material, including variousmetals and metal alloys, ceramics, glasses and the like. Where thecomposition gradient is also being used to provide a density gradient,the use of various high atomic weight metals may be useful, includingthose in Groups IVB-VIIB of the periodic table.

A density gradient may be established in any suitable manner, includingthat described above where a powder of the first material 32 is mixedwith a powder of another material, such as second material 42, and thencompacted to a predetermined density, such as the full theoreticaldensity of the mixture of these materials, to form a powder compact. Adensity gradient may also be established in the gradient portion 70 bycompacting a powder of the first material 32 to a density different thanthat of the first material 32 of the core member 30, including a densitythat is greater than or less than the density of the core member 30. Inone embodiment, the core member 30 may comprise a powder compact of apowder of the first material 32 that is compacted to full theoreticaldensity, and a gradient portion 70 layer may comprise a powder compactof the powder of the first material 32 that is compacted to less thanfull theoretical density. In another embodiment, the core member 30 maycomprise a powder compact of a powder of the first material 32 that iscompacted to less than full theoretical density, and gradient portion 70or layer may comprise a powder compact of a powder of the first material32 that is compacted to a higher density, including full theoreticaldensity.

The gradient portion 70 having the composition gradient or the densitygradient, or a combination thereof, of the first material 32 may extendfrom an outer portion 35 proximate the outer member 40 toward an innerportion 37 away from the outer member 40 either as a single layer orregion as shown in FIG. 2, or a plurality of discrete layers orcompositional steps, as illustrated in FIG. 3. In one embodiment, thegradient portion 70 may include a decreasing amount or a decreasingdensity, or a combination thereof, of the first material 32 from theinner portion 37 to the outer portion 35. For example, in FIG. 2, thecore member 30 comprises 100 weight percent of the first material 32,such as a nanomatrix of aluminum having magnesium or magnesium alloyparticles dispersed therein. Gradient portion 70 includes three discretelayers having different compositions. The first layer 80 may comprise,for example, a substantially spherical powder compact of a homogeneouspowder mixture that comprises 75% by weight of the first material 32 and25% by weight of the second material 42. The second layer 82 maycomprise, for example, 50% by weight of the first material 32 and 50% byweight of the second material 42. The third layer 84 may comprise, forexample, 25% by weight of the first material 32 and 75% by weight of thesecond material 42. The outer member 40 comprises 100% by weight of thesecond material. In this embodiment, the composition gradient or thedensity gradient, or a combination thereof, varies in discrete stepsfrom the inner portion 37 to the outer portion 35 corresponding tolayers that provide a plurality of discrete compositional and densitysteps, each having a different composition and density as describedabove.

In another example, the composition gradient or the density gradient, ora combination thereof, of the first material in the core member variescontinuously from the inner portion to the outer portion as illustratedin FIG. 2. The amount of the first member may vary, for example, from100% by weight of the first material in the inner portion 37 of coremember 30 to 0% by weight in the outer portion 35. Correspondingly, theamount of the other material, such as second material 42, may vary, forexample, from 100% by weight of the second material in the outer portion35 to 0% by weight in the inner portion 37. In this example, thecorrodible core member 30 also comprises a gradient portion 70 having acomposition gradient or a density gradient, or a combination thereof, ofthe second material 42 in the corrodible core member 30 from the outerportion 35 proximate the outer member 40 toward the inner portion 37.

The outer member 40 may be configured to have a thickness, eitheruniform or a variable, sufficient to provide a predetermined workingtime of the downhole article 10, including a predetermined working timein the predetermined wellbore fluid 24, whereupon the corrosion rate ofthe second material 42 in the predetermined wellbore fluid thins theouter member sufficiently that the predetermined wellbore fluid contactsthe first material 32 and begins to rapidly corrode the core member 30,including the gradient portion 70 therebetween. For example, thecorrosion of the outer member 40 may proceed substantially uniformly atthe second corrosion rate over all or a portion of the surface 44 of theouter member 40 until the predetermined wellbore fluid 24 breaches theouter member 40 and contacts the first material 32 of core member 30,including the gradient portion 70 disposed therebetween. In anotherexample, the outer member 40 may include an access point 90, or aplurality of access points 90, that is configured to provide access ofthe predetermined wellbore fluid 24 through the outer member 40 to thecore member 30 in order to corrode the first material 32 of thecorrodible core member 30 in response to a predetermined wellborecondition as illustrated in FIGS. 4-12. The wellbore condition mayinclude any suitable condition that may be used to provide access of thepredetermined wellbore fluid 24 to the corrodible core member 30. In oneembodiment, the access point 90 may include a localized thinning of theouter member 40 and second material 42, either by providing a recess inthe surface 44 of the outer member or a protrusion of the corrodiblecore member 30 as shown in FIG. 4 and the wellbore condition may includeplacing the predetermined wellbore fluid in contact with the accesspoint 90 for a time sufficient to enable the predetermined wellborefluid 24 to corrode the thickness of the second material 42 at theaccess point 90. In another embodiment, the access point 90 may alsocomprise a different access point material 92 that may provide accessthrough the outer member 40 in response to a wellbore condition, or achange in a wellbore condition, other than the predetermined wellborefluid 24. For example, the wellbore condition may comprise heat orpressure, or a combination thereof, sufficient to alter a property ofthe access point 90, such as by a phase transformation, includingmelting, or a change in the mechanical properties, sufficient to enablethe predetermined wellbore fluid 24 to access the core member 40. Inanother embodiment, the access point 90 may comprise a check valve 94and enable access of the predetermined wellbore fluid 24 in response toa wellbore condition that includes a change in pressure. Any suitablewellbore conditions may also be used to provide access of thepredetermined wellbore fluid 24 through the outer member 40 to the coremember 30 through the access point 90. In an exemplary embodiment, theaccess point 90 may include at least one of the thickness difference, acompositional difference or a density difference of the second material40 of the outer member 40 that is sufficient to provide access of thepredetermined wellbore fluid 24 to the core member 30 in response to awellbore condition, or a change in a wellbore condition.

In an exemplary embodiment, the at least one corrodible core member 30and the at least one outer member 42 may comprise a plurality of coremembers having a corresponding plurality of outer members disposedthereon, wherein the respective core members 30 and associated outermembers 40 are nested within one another to form an alternatingarrangement of core members 30 and outer members 40 as illustrated inFIGS. 11 and 12. In the embodiment of FIG. 11, a plurality of hollowspherical core members 30 are nested within one another to form analternating arrangement of diverter balls 12 comprising core members 30with outer members 40. Each of the core members 30 and correspondingouter members 40 may be formed sequentially using methods describedherein so that the innermost outer member 40/core member 30 may beencompassed by one or more successively larger outer members 40/coremembers 30. While the same shapes may be nested within one another, suchas the plurality of diverter balls 12 illustrated in FIG. 11, as well asa plurality of cylindrical plugs (not shown) or a plurality of nestedsleeves 22 or ball seats 14 (not shown), it is also possible to nestdissimilar shapes within one another. In the exemplary embodimentillustrated in FIG. 12, a cylindrical plug 16 may have an article 10having another shape nested therein, such as a diverter ball 12.Likewise, a cylindrical plug 16 may have a diverter ball 12 or pluralityof diverter balls 12 nested therein (not shown). These configurationsprovide an article 10 that may be selectively corroded to reconfigurethe article into another article 10 that may be used for a subsequentwellbore operation without the necessity of running in the second orsubsequent article 10. For example, a plurality of nested balls 12 maybe used such that upon completion of a wellbore operation at aparticularly level in the wellbore the outermost outer member 40/coremember 30 may be removed and the diameters may be selected such that theremaining article 10 can pass through a ball seat to a lower level ofthe wellbore, for example. Of course, one of ordinary skill willunderstand that the opposite arrangement may also be affected, such thatremoval of the outermost outer member 40/core member 30 will enable theball to be moved upwardly through a ball seat to a portion of thewellbore closer to the earth's surface.

While the arrangement described above is useful in many applications,including those described, a reverse arrangement of the first material32 and second material 42 is also possible as illustrated in FIG. 10,where the first material comprises the outer member 50 and the secondmaterial 42 comprises the core member 60 such that the outer member 50may be rapidly corroded in a predetermined wellbore fluid 24 to exposethe core member 60 that has a much lower corrosion rate. As such, thecomposite downhole article 10 may include at least one corrodible outermember 50 comprising a first material 32 as described herein that iscorrodible in a wellbore fluid 24 at a first corrosion rate and at leastone core member 60 disposed within the outer member 50 and comprising asecond material 42 that is corrodible in the predetermined wellborefluid 24 at a second corrosion rate, wherein the corrodible outer member50 has a gradient portion 70 having a composition gradient or a densitygradient, or combination thereof, and wherein the first corrosion rateis substantially greater than the second corrosion rate as describedherein. Such a configuration may be desirable, for example, to positionan article 10 in the wellbore in a certain location by using the outermember to orient the article 10 within the wellbore, followed byexposure to the predetermined wellbore fluid 24 to remove the outermember 50 and leave the core member in a specific location ororientation.

Referring to FIG. 13, a method 300 of making composite downhole articles10 as described herein is disclosed. The method 300 generally includesforming 310 at least one corrodible core member 30 comprising a firstmaterial 32 that is corrodible in a wellbore fluid 24 at a firstcorrosion rate and disposing 220 at least one outer member 40 on thecore member 30, the outer member 40 comprising a second material 42 thatis corrodible in the wellbore fluid at a second corrosion rate, whereinthe corrodible core member 30 has a composition gradient or a densitygradient, or a combination thereof, and wherein the first corrosion rateis substantially greater than the second corrosion rate.

The corrodible core member 30 may have any suitable configuration,including size and shape, as described herein. Forming 310 of thecorrodible core member 30 may be performed using any suitable formingmethod, including pressing and dynamic forging of various powdercompacts, particularly powder compacts of various coated metallicpowders as described herein.

Forming 310 may include forming an unsintered or precursor powdercompact 100 as the corrodible core member 30. An unsintered powdercompact 100 (FIG. 18) may be formed, for example, by employing variouspowder compaction methods such as pressing, forging, extrusion,isostatic pressing and the like. Generally, the powder compaction toform an unsintered or precursor powder compact 100 will be performedwithout providing an external source of heat for heating the powderparticles during compaction, or alternately, by heating the powderduring compaction to a temperature that is substantially lower than amelting temperature of the material selected for the metallic coatinglayer, so that there is substantially no solid-state interdiffusionbetween adjacent powder particles. Unsintered powder compacts 100 mayform mechanical bonds, for example, between the metallic coating layersof adjacent powder particles sufficient to retain a compacted shape ofthe corrodible core member 30. Unsintered powder compacts 100 willgenerally have a predetermined porosity or density, with the amount ofporosity or density determined by factors associated with thecompaction, such as the compaction pressure and time and the nature ofthe metallic powder used to form the compact. In one embodiment, theunsintered powder compact 100 may be formed by compacting a powdercomprising a plurality of metallic powder particles, each powderparticle comprising a particle core, the particle core comprises a corematerial comprising Mg, Al, Zn, Fe or Mn, or alloys thereof, or acombination thereof, and a metallic coating layer disposed on theparticle core, wherein compacting causes the metallic coating layers ofadjacent particles to form mechanical bonds to one another sufficient toform and retain the shape of the powder compact as illustrated in FIG.18.

Forming 310 may also include forming a sintered powder compact 200 asthe corrodible core member 30. A sintered powder compact 200 may includesintering to achieve substantially full theoretical density of thepowder compact, as well as partial sintering to achieve less than fulltheoretical density of the powder compact, including partial sinteringto achieve a predetermined porosity or density. Sintered powder compactswill generally be characterized by interdiffusion, such as solid-stateinterdiffusion, between the metallic coating layers of adjacent powderparticles such that chemical or metallic bonds are formed between them.A sintered powder compact may be formed, for example, by employingvarious powder compaction methods such as pressing, rolling, forgingincluding dynamic forging, extrusion or isostatic pressing including hotisostatic pressing, or a combination thereof, and the like. Generally,powder compaction to form a sintered powder compact will be performed inconjunction with providing an external source of heat for heating thepowder particles during compaction, and may including heating the powderduring compaction to a temperature near the melting temperature of thematerial selected for the metallic coating layer. In some embodiments,this may include heating the powder to a temperature just below amelting temperature of the metallic coating layer material, and in otherembodiments may even include heating the powder to temperature that isslightly above a melting temperature of the metallic coating layermaterial. In an exemplary embodiment, forming 310 the sintered powdercompact and corrodible core member 30 comprises forming asubstantially-continuous, cellular nanomatrix comprising a nanomatrixmaterial that includes a plurality of dispersed particles comprising aparticle core material that comprises Mg, Al, Zn, Fe, or Mn, or alloysthereof, or a combination thereof, dispersed in the cellular nanomatrix,and a bond layer extending throughout the cellular nanomatrix betweenthe dispersed particles as illustrated in FIG. 17.

The gradient portion 70 and the associated composition or densitygradient, or combination thereof, may be disposed between the corrodiblecore member 30 and the outer member 40 by any suitable method. It may beformed integrally with the corrodible core member 30, or as a separategradient portion 70 or member that is disposed between the corrodiblecore member 30 and the outer member 40 prior to the outer member 40being disposed on the corrodible core member 30, or by depositing alayer having the composition or density gradient, or combination thereofon the corrodible core member 30 prior to disposition of the outermember 40 thereon, for example. Forming 310 the corrodible core member30 may include establishing the composition gradient or the densitygradient, or a combination thereof, of the first material 32 from anouter portion 35 of the core member 30 proximate the outer member 40toward an inner portion 37 of the core member 30 away from the outermember 40 as illustrated in FIGS. 2 and 3, for example.

In one embodiment, forming 310 includes establishing the compositiongradient or the density gradient, or a combination thereof, of the firstmaterial 32 from an outer portion 37 proximate the outer member 40toward an inner portion 35 away from the outer member 40 by varying thecomposition gradient or the density gradient, or a combination thereof,continuously from the inner portion 37 to the outer portion 35 as shownin FIG. 2. This may be accomplished, for example, by varying the densityof the first material 32 continuously in the gradient portion 70.Various forms of spray forming and fusion of a powder of the firstmaterial 32 may be employed to vary the density continuously, such aslaser sintering of a precursor compact 100, laser direct deposition orcladding, stereolithography and fused deposition modeling depositionmethods. This may include, for example, progressive laser fusion of apowder of the first material 32 having a single powder size withcontinuously varying energy or power to provide greater and lesserdegrees of fusion and bonding of the powder particles and a continuouslyvarying density. In another exemplary embodiment, the composition ordensity, or a combination thereof, of the first material 32 may bevaried continuously from the inner portion 37 to the outer portion 35.Various forms of spray forming and fusion of at least two powders sizesof the first material 32 may be employed to vary the density orcomposition, or a combination thereof, by continuously varying theamount of the two powder sizes provided using a fixed or a variableenergy or power density. In yet another exemplary embodiment, this maybe performed by varying the composition of the first material 32 in thegradient portion 70 by incorporation of a continuously varying amount ofanother material, such as the second material 42 in the first material32 in the gradient portion 70. For example, a continuously varyingamount by weight from 100% second material 42/0% first material 32 inthe outer portion 35 proximate the outer member 40 to 0% second material42/100% first material 32 in the inner portion 37. Various forms ofspray forming and fusion of powders of the first material and the othermaterial, such as second material, may be employed to vary thecomposition continuously, such as laser fusion of the powders as theyare simultaneously applied in continuously varying proportions using afixed or a variable energy or power density. Various known methods ofensuring uniform coverage of the material or materials being depositedmay be employed, including rotating or rastering of the substrate duringdeposition and laser fusion of a sprayed powder, or alternately,rastering of a spayed powder and laser over the surface of the substrateduring deposition.

In another embodiment, forming 310 includes establishing the compositiongradient or the density gradient, or a combination thereof, of the firstmaterial 32 from an outer portion 35 proximate the outer member 40toward an inner portion 37 away from the outer member 40 by varying thecomposition gradient or the density gradient, or a combination thereof,in discrete steps or layers from the inner portion 37 to the outerportion 35 as shown in FIG. 3. The density or composition, or acombination thereof, in each of the steps may be varied using the firstmaterial 32, or a combination of the first material 32 and anothermaterial, such as the second material 42, by any suitable method, suchas the methods using laser fusion of powders described above. In eachstep or layer, the density or composition, or combination thereof, maybe constant or may vary continuously.

Disposing 320 at least one outer member 40 on the core member 30 may beperformed by any suitable method. In one embodiment, disposing 320 theouter member 40 on the core member 30 may include disposing a powdercompact of the second material 42 on the core member 30. This may beperformed, for example, by compacting a form or plurality of forms ofthe second material 42 that may be used to encompass the corrodible coremember 30. For example, if the corrodible core member 30 issubstantially spherical, the outer member 40 may comprise two hollowhemispherical powder compact members 40 that are sized to dispose theirinner surfaces against an outer surface of the corrodible core memberand be joined along their adjoining peripheral edges by a joint as shownin FIG. 1. In another embodiment, disposing 320 the outer member 40 onthe core member 30 may include depositing a layer of the second material42 on the core member 30. A layer of the second material 42 may bedeposited by any suitable deposition method, including dipping in amolten metal bath, plating including electroplating and electrolessplating, sputtering and other thin film deposition techniques, cladding,compacting a powder, thermal spraying, or laser fusion of a powder ofthe second material 42 on the outer surface or portion of the corrodiblecore member, or a combination thereof.

Referring to FIG. 13, method 300 may also optionally include forming 330an access point 90 on the outer member, the access point 90 configuredto provide access of a wellbore fluid to the core member in response toa change in a wellbore condition as described herein. Forming 330 of theaccess point 90 on the outer member 40 may be performed by any suitableforming method. Forming 330 may be performed integrally in conjunctionwith disposing 320 the outer member 40 on the core member 30, or by anadditional forming operation or operations. For example, where theaccess point 90 comprises a localized thinning of the second material42, this may be accomplished by design of the core member 30 and/orouter member 40. Alternately, it may be performed by chemical,mechanical or other removal of second material 42 from the outer member40. Chemical removal may be accomplished by chemical or electrochemicalmilling, etching or other chemical removal methods, and may include theuse of photolithographic masking or patterning techniques to define theform or shape of the access point 90 followed by suitable materialremoval by etching or other material removal techniques to form theaccess point. Mechanical removal may be accomplished by machining,drilling, grinding or other material removal methods.

As described above, a reverse arrangement of the first material 32 andsecond material 42 is also possible as illustrated generally in FIGS. 10and 14, and may be formed by a method 400 of making a composite downholearticle 10 that includes forming 410 at least one core member 60comprising a second material 42 that is corrodible in a wellbore fluidat a second corrosion rate and disposing 420 at least one corrodibleouter member 50 on the core member 60 comprising a first material 32that is corrodible in the predetermined wellbore fluid 24 at a firstcorrosion rate, wherein the corrodible outer member 50 has a compositiongradient or a density gradient, or a combination thereof, and whereinthe first corrosion rate is substantially greater than the secondcorrosion rate. In this configuration, the core member 60 of the secondmaterial 42 may be formed by any suitable fabrication method, includingcasting, forging, machining or various powder compaction methods, or acombination thereof.

Forming 410 the corrodible outer member 50 may include establishing thecomposition gradient or the density gradient, or a combination thereof,of the first material 32 from an inner portion 37 proximate the coremember 60 toward an outer portion 35 away from the core member 60.Establishing the composition gradient or the density gradient, or acombination thereof, of the first material 32 from an inner portion 37proximate the core member 60 toward an outer portion 35 away from thecore member 60 may include varying the composition gradient or thedensity gradient, or a combination thereof, continuously as describedherein from the inner portion 37 to the outer portion 35. Alternately,or in combination therewith, establishing the composition gradient orthe density gradient, or a combination thereof, of the first material 32from an inner portion 37 proximate the core member 60 toward an outerportion 35 away from the core member 30 may include varying thecomposition gradient or the density gradient, or a combination thereof,in discrete steps or layers from the inner portion 37 to the outerportion 35. Within each step, the composition gradient or the densitygradient may be constant or vary continuously as described herein. Asdescribed herein the gradient portion 70, including the compositiongradient or the density gradient, or a combination thereof, may beformed as a powder compact of the first material 32, or a combination ofthe first material and another material, including the second material42, as described herein. In one embodiment, the method 400 and forming410 may include establishing a composition gradient or a densitygradient, or a combination thereof, of another material, including thesecond material 42 of the core member 60, in the corrodible outer member50 from the inner portion 37 proximate the core member toward the outerportion 35, analogous to combinations of first material 32 and secondmaterial 42 described elsewhere herein.

In one embodiment, disposing 420 the corrodible outer member 50 on thecore member 30 includes disposing a powder compact of the first material32 on the core member 60. The powder compact of the first material 32may be formed directly on the core member 60 using any of the powderapplication or compaction methods disclosed herein, or alternately, maybe formed separately as a single piece or in multiple pieces, anddisposed on the core member 60 by any suitable disposition method,including the methods disclosed herein, for attaching, joining orotherwise disposing the second material 42 on the first material 32.

In an exemplary embodiment, a method 500 of using a composite downholearticle is disclosed as illustrated in FIG. 15. The method 500 includesforming 510 a composite downhole article that includes a first member30, 50 comprising a first corrodible material 32 that is corrodible in apredetermined wellbore fluid 24 at a first corrosion rate and a secondcorrodible member 40,60 comprising a second material 42 that iscorrodible in the wellbore fluid at a second corrosion rate, wherein thefirst corrodible member has a gradient portion 70 having compositiongradient or a density gradient, or a combination thereof, and whereinthe first corrosion rate is substantially greater than the secondcorrosion rate. Forming 510 may include or employ, for example, eitherof the method 300 or method 400 of making a composite downhole article10. The method 500 also includes using 520 the article 10 to perform afirst wellbore operation; exposing 530 the article to the predeterminedwellbore fluid 24; and selectively corroding 540 the first corrodiblemember 30,50. The article used in method 500 may include any suitabledownhole article 10, particularly various downhole tools and components.

In one embodiment, the downhole article 10 may include variousconfigurations of diverter balls 12, plugs 16 or disks 20 as disclosedherein, wherein using 520 the article to perform a predeterminedwellbore operation includes completely or partially closing an orificein conjunction a fracturing, completion or production operation. Thedownhole article 10 has an outer member 40 that comprises a thin layeror coating of the second material 42 sufficient to close the desiredorifice and resist the predetermined wellbore fluid 24 for a timesufficient to perform the predetermined wellbore operation, such asfracturing an earth formation. The outer member 40 and predeterminedwellbore fluid 24 may be selected so that upon occurrence of acondition, such as, for example, passage of time sufficient forcompletion of the predetermined wellbore operation the predeterminedwellbore fluid 24 has dissolved the outer member 40 sufficiently to gainaccess to the core member 30, whereupon the core member 30 is rapidlycorroded by the predetermined wellbore fluid 24 causing any remainingportion of the outer member to collapse or disintegrate, therebyremoving the diverter ball 12, plug 16 or disk 20 and opening theorifice. Other wellbore conditions may also be employed in anycombination, including increasing a temperature and/or pressure of awellbore fluid, insertion of another substance, such as another wellborefluid to selectively increase the second corrosion rate of the secondmaterial 42 to facilitate its corrosion to provide access of the fluidto the first material 32.

In another embodiment, the downhole article 10 may include a tubularsection 23 that may be used to form a portion of a casing of a wellborehaving one or more portions of the tubular wall that include features 25that includes a core member 30 and outer member 40 to define a feature,such as a through-hole 91 or opening, a blind hole 93 or blind opening,conduit, passage, groove 95, protruding rib 97, shoulder 99 or otherfeature. Using 520 the article 10 to perform a predetermined wellboreoperation may include any suitable wellbore operation, such as use of atubular section 23 as a conduit for fluids, slicklines, wirelines,tools, components or other wellbore articles through the tubular sectionfor various purposes associated with fracturing, completion orproduction operations. The outer member 40 and wellbore fluid 24 may beselected so that upon occurrence of a condition, such as, for example,passage of time sufficient for completion of the predetermined wellboreoperation the wellbore fluid dissolves the outer member 40 sufficientlyto access the core member 30, whereupon the core member 30 is rapidlycorroded by the wellbore fluid 24 causing any remaining portion of theouter member to collapse or disintegrate, thereby exposing the featuredefined in the tubular section. This may be used, for example, to createan opening or multiple openings through the tubular section 23 analogousto a perforating operation, or to open a conduit such as might be usedfor a number of completion or production operations, including afracturing operation. Exposure of a shoulder 95 or protruding rib 97 onan internal surface may be used, for example to provide a seat for asleeve, ball or plug.

In yet another embodiment, the downhole article 10 may include a hollowcylinder that may be inserted, for example, within a casing and used asa sleeve 22 or seat, including a ball seat 14 or plug seat 18, havingone or more portions of the hollow cylinder comprising a core member 30and outer member 40 as disclosed herein. Using 520 may include anysuitable use of the hollow cylinder, including as various fixed orsliding sleeves that may be used within a casing, such as sleeves thatare use to conceal or reveal an opening or conduit in a casing, orvarious cylindrical seats that may be used with a ball 12 or plug 16 toopen or close the wellbore for various purposes associated withfracturing, completion or production operations. The outer member 40 andpredetermined wellbore fluid 24 may be selected so that upon occurrenceof a condition, such as, for example, passage of time sufficient forcompletion of a predetermined wellbore operation the predeterminedwellbore fluid 24 has dissolved the outer member 40 sufficiently toaccess the core member 30, whereupon the core member 30 is rapidlycorroded by the wellbore fluid 24 causing any remaining portion of theouter member 40 to collapse or disintegrate, thereby removing the hollowcylinder from the wellbore.

Exposing 530 the article to the predetermined wellbore fluid 24 mayinclude exposing the article 10 to any predetermined wellbore fluid 24that is suitable for corrosion of the corrodible first material 32 andsecond material 42 as described herein. In one embodiment, exposing 530may include immersing an exposed surface of the second material 42 inthe wellbore fluid for a time sufficient to corrode through the secondmaterial 42 to the gradient portion 70, wherein the first material 32 inthe gradient portion 70 begins to rapidly corrode and the first member30, including the gradient portion 70, may be rapidly removed. Suitablewellbore fluids 24 may include water, various aqueous solutions, brinesor acids, including organic or inorganic acids, or a combinationthereof. In another embodiment, exposing 530 the downhole article 10 tothe wellbore fluid 24 comprises opening an access point 90 in the secondmember 40 in response to a wellbore condition to allow the wellborefluid to access the first corrodible member as described herein.

Selectively corroding 540 may include completely corroding the firstcorrodible member 30 such that the downhole article 10 is completelyremoved from the wellbore by the predetermined wellbore fluid 24.Alternately, selectively corroding 540 may comprise removing a portionof the downhole article 10. This may include, for example, corroding thefirst corrodible member 30 as described herein to alter the shape orsize of the article 10. In one embodiment, where the article 10comprises a plurality of nested articles, such as a plurality of nestedballs 12, as described herein, selectively corroding 540 may includeremoving an outermost layer, such as an outermost ball 12, so that thesize (e.g., diameter) of the article 10 is reduced and the remainingportion may pass through a seat to another section of the wellbore,either closer to or farther from the earth surface to be seated inanother seat. Selectively corroding 540 may be repeated to successivelyremove nested articles 10 and reduce the size, such as the diameter of aball 12, allowing the remaining portion to be progressively movedthrough a ball seat to another section of the wellbore, either closer toor farther from the earth surface to be seated in another seat.

While preferred embodiments have been shown and described, modificationsand substitutions may be made thereto without departing from the spiritand scope of the invention. Accordingly, it is to be understood that thepresent invention has been described by way of illustrations and notlimitation.

1. A composite downhole article, comprising: at least one corrodiblecore member comprising a first material that is corrodible in a wellborefluid at a first corrosion rate; and at least one outer member disposedon the core member and comprising a second material that is corrodiblein the wellbore fluid at a second corrosion rate, wherein the corrodiblecore member has a composition gradient or a density gradient, or acombination thereof, and wherein the first corrosion rate issubstantially greater than the second corrosion rate.
 2. The article ofclaim 1, wherein the composition gradient or the density gradient, or acombination thereof, of the first material extends from an outer portionproximate the outer member toward an inner portion away from the outermember.
 3. The article of claim 2, wherein the composition gradient orthe density gradient, or a combination thereof, comprises a decreasingamount or a decreasing density, or a combination thereof, of the firstmaterial from the inner portion to the outer portion.
 4. The article ofclaim 3, wherein the composition gradient or the density gradient, or acombination thereof, of the first material in the core member variescontinuously from the inner portion to the outer portion.
 5. The articleof claim 3, wherein the composition gradient or the density gradient, ora combination thereof, varies in discrete steps from the inner portionto the outer portion.
 6. The article of claim 2, wherein the corrodiblecore member also comprises a composition gradient or a density gradient,or a combination thereof, of the second material in the corrodible coremember from the outer portion proximate the outer member toward theinner portion.
 7. The article of claim 1, further comprising an accesspoint configured to provide access of the wellbore fluid to the coremember in response to a wellbore condition.
 8. The article of claim 1,wherein the access point comprises at least one of a thicknessdifference, a compositional difference or a density difference of thesecond material that is sufficient to provide access of the wellborefluid to the core member in response to a wellbore condition.
 9. Thearticle of claim 1, wherein the corrodible core member is substantiallyspherical and the outer member is a substantially spherical layerdisposed on the core member.
 10. The article of claim 1, wherein thecorrodible core member is substantially cylindrical and the outer membercomprises a layer disposed on the core member.
 11. The article of claim9, wherein the core member comprises a hollow sleeve disposed about alongitudinal axis and the outer member comprises a layer disposed on thecore member.
 12. The article of claim 1, wherein the first materialcomprises a powder metal compact comprising a substantially-continuous,cellular nanomatrix comprising a nanomatrix material; a plurality ofdispersed particles comprising a particle core member material thatcomprises Mg, Al, Zn, Fe or Mn, or alloys thereof, or a combinationthereof, dispersed in the cellular nanomatrix; and bond layer extendingthroughout the cellular nanomatrix between the dispersed particles. 13.The article of claim 1, wherein the second material comprises a powdercompact.
 14. The article of claim 1, wherein the second materialcomprises a metal, polymer or ceramic, or a combination thereof.
 15. Thearticle of claim 1, wherein the at least one corrodible core member andthe at least one outer member comprise a plurality of core membershaving a corresponding plurality of outer members disposed thereon,wherein the respective core members and associated outer members arenested within one another to form an alternating arrangement of coremembers and outer members.
 16. A composite downhole article, comprising:at least one core member comprising a second material that is corrodiblein a wellbore fluid at a second corrosion rate; and at least onecorrodible outer member disposed on the core member and comprising afirst material that is corrodible in the wellbore fluid at a firstcorrosion rate, wherein the corrodible outer member has a compositiongradient or a density gradient, or a combination thereof, and whereinthe first corrosion rate is substantially greater than the secondcorrosion rate.
 17. The article of claim 16, wherein the corrodibleouter member has a composition gradient of the first material from anouter portion proximate an outer surface toward an inner portionproximate the core member.
 18. The article of claim 17, wherein thecomposition gradient of the first material comprises a decreasing amountof the first material from the outer portion to the inner portion. 19.The article of claim 18, wherein the composition gradient of the firstmaterial varies continuously from the outer portion to the innerportion.
 20. The article of claim 18, wherein the composition gradientof the first material varies in discrete steps from the outer portion tothe inner portion, each step having a substantially constant amount ofthe first material.
 21. The article of claim 17, wherein the outermember also comprises a composition gradient of the second material inthe outer member from the inner portion proximate the core member towardthe outer portion, and the composition gradient of the second materialcomprises a decreasing amount of the second material from the innerportion toward the outer portion.
 22. The article of claim 16, whereinthe first material comprises a powder metal compact comprising asubstantially-continuous, cellular nanomatrix comprising a nanomatrixmaterial; a plurality of dispersed particles comprising a particle coremember material that comprises Mg, Al, Zn, Fe or Mn, or alloys thereof,or a combination thereof, dispersed in the cellular nanomatrix; and bondlayer extending throughout the cellular nanomatrix between the dispersedparticles.
 23. The article of claim 16, wherein the second materialcomprises a powder compact.
 24. The article of claim 16, wherein thesecond material comprises a metal, polymer or ceramic, or a combinationthereof.